The present invention relates, in general, to an apparatus for levitated guidance of a material web in a travel direction, and in particular to a blow box of a type having a planar surface portion positioned in facing relationship to the material web, and two slots positioned in space-apart relationship laterally next to curved edge zones of the surface portion and extending transversely to the travel direction, with the surface portion being formed at a central location between the slots with at least one row of orifice openings or nozzles in communication with a fluid chamber which is connected to a source for supply of fluid, e.g., air.
German patent specification DE 20 08 804 describes a blow box of this type. The surface portion is formed with one or more rows of nozzles and is connected via the curved edge zones to side walls which are parallel to one another and extend into the air chamber. The side walls terminate in end portions which are bent by 90.degree. with respect to the side walls and extend parallel to the surface portion, with the end portions terminating in longitudinal edges which form in conjunction with the surface portion the slots. Linear jet streams exit the slots in a converging fashion in the direction to the material web.
As the slots as well as the nozzles in the surface portion are connected directly to the air chamber that is in communication with the air source, blowing air is discharged through the slots and the nozzles at a same velocity. Moreover, a pronounced, wavy pressure curve is established between the material web and the blow box, so that the demand for blowing air is comparably high. As a consequence, the units for supply of blowing air must be accordingly dimensioned. This conventional blow box suffers the further drawback that the converging jet streams exiting the slots contribute only slightly to the drying effect of the material web in the area between these jet streams. The drying action is predominantly realized by the marginal layers of the jet streams which support the material web in, or in opposition to, the travel direction of the material web.
Converging jet streams generate a great pressure area when the material web is at comparably slight distance from the surface portion. However, an increasing distance of the material web to the surface portion results in a decrease of the pressure area until the jet streams intersect at which point the pressure approaches zero. Thus, the capability of the jets to carry or support the material web deteriorates. Although, an increase of the dynamic pressure of the blowing air could conceivably prevent a contact between the material web and the surface portion of the blow box; However, such a measure is accompanied by the drawback that an even greater amount of air is required, without positively affecting the actual drying action.
In view of the unsteady carrying behavior as displayed by the converging jets exiting the slots, there is a risk that the marginal areas of the material web begin to flutter because the blowing air flows off laterally so that the dynamic pressure of the blowing air is reduced. Thus, the marginal areas of the material web are poorly dried, and, moreover, there is a risk that the material web may touch the surface portion of the blow box.